tests: make test-extension.t's baduisetup.py test fail even if bdiff exists A mercurial.bdiff module has existed at some point and when I have built at an old version, some .pyc file or something seems to stick around and test-extension.t fails. Let's remove the reference to bdiff and simplify the test. Differential Revision: https://phab.mercurial-scm.org/D1162